Answer 4.5

a) Briefly describe the setup of a linear quadrupole analyzer.

A linear quadrupole mass analyzer consists of four hyperbolically or cyclindrically shaped rod electrodes extending in the z-direction and mounted in a square configuration (xy-plane). The pairs of opposite rods are each held at the same potential which is composed of a DC voltage U and a radiofrequency (RF) voltage V with the frequency f.

b) Give a short explanation of the working principle of a linear quadrupole analyzer.

As an ion enters the quadrupole assembly in z-direction, an attractive force is exerted on it by one of the rods with its charge actually opposite to the ionic charge. If the voltage applied to the rods is periodic, attraction and repulsion in both the x– and y-directions are alternating in time, because the sign of the electric force also changes periodically in time.
For a given set of DC voltage U and a radiofrequency (RF) voltage the overall ion motion can result in a stable trajectory causing ions of a certain m/z value or m/z range to pass the quadrupole.
The path stability of a particular ion is defined by the magnitude of the RF voltage V and by the ratio U/V.

c) What advantages do triple quadrupole mass spectrometers offer in addition to single quadrupole instruments?

In triple quadrupole instruments (QqQ), Q1 and Q3 are operated independently as MS1 and MS2, respectively, making MS/MS a straightforward matter.